QFileSystemModel: fix permission mangling

QFileSystemModel: fix permission mangling

The old code masked out write flags before returning permissions
from permissions() or data(FilePermissions) in order to force
QFileDialog to disable the rename and delete actions. This was to
fix Task 143519, but introduced QTBUG-20503.

Instead, revert to the pre-143519-bugfix code and do the necessary
check in QFileDialog directly.

Also add a testcase for 143519.

Reported-by: Gilles Pascual
Task-number: QTBUG-20503
Task-number: 143519

Change-Id: I140109341c0ed40722e3aac4327c2a740fb014c2
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>